1. Field of the Invention
The present invention relates to a device for coating a polyimide of a semiconductor device, and more particularly, to a polyimide coating device including a means for preventing the unnecessary consumption of the polyimide.
2. Description of the Related Art
A polyimide is one of materials used as an interlayer insulating film in a semiconductor device manufacturing process. The polyimide is a fluid in contrast to other interlayer insulating films such as oxide or nitride films.
Since the polyimide is a fluid, when a structure having a severe step difference is coated with the polyimide, the surface of the step difference portion is smoothed over. As a result, the step difference can be reduced, and a narrow channel having a high aspect ratio can be filled without forming a void. Once hardened, a polyimide does not change its material properties even at temperatures over 500.degree. C.
The polyimide characteristics can vary with a particular polyimide coating application. That is, the smoothing characteristics are not evident where poor coatings exist, such as when foam or small gas bubbles are resident in a coated polyimide film.
Referring to FIG. 1, a conventional polyimide coating device generally includes a polyimide storage tank 10 and a plurality of pipe lines 16, 20 and 22 connected to the polyimide storage tank 10, each pipe line having a different function.
More specifically, the polyimide storage tank 10 is an opaque airtight tank containing a vessel 14 filled with a polyimide 12. A polyimide supply pipe 16 is connected to the vessel 14 for supplying the polyimide to a reaction chamber (not shown) via a pressure differential. In effect, the polyimide is forced into the polyimide supply pipe 16 by increasing the inner pressure of the storage tank 10. A gas, for example, nitrogen, is used as a means for increasing the pressure inside the storage tank 10, and accordingly, a nitrogen supply pipe 22 is connected to the storage tank 10. In addition, an air valve 24 communicates with the storage tank 10 to keep the pressure of the storage tank 10 constant.
An intermediate valve 18 is installed in the polyimide supply pipe 16 for blocking and discharging the polyimide 12 supplied from the storage tank 10. A discharge pipe 20 connects to the intermediate valve 18 for discharging the polyimide 12. The intermediate valve 18 serves as an interlocking valve so that the supply and discharge of the polyimide is performed consecutively. That is, when the intermediate valve 18 operates to supply the polyimide 12 to the reaction chamber, a passage leading to the discharge pipe 20 closes. On the other hand, when the intermediate valve 18 opens the discharge pipe 20 to discharge the polyimide 12, a passage leading to the supply pipe 16 closes.
With this conventional configuration, since the storage tank 10 is opaque, it is difficult to determine the amount of polyimide 12 remaining in the vessel 14. Thus, the conventional polyimide coating device monitors the coating frequency, or number of coating applications performed, using a counter. If the coating frequency reaches a predetermined value, the polyimide 12 stored in the storage tank 10 is considered low and the polyimide vessel 14 is then replaced with a new one. In general, when the counter reaches a value of 120, the polyimide 12 in the vessel 14 is considered low and the vessel is replaced.
However, since the polyimide coating amount for each application can vary slightly, the amount of polyimide 12 remaining in the vessel 14 may be sufficient even after reaching the designated count value. On the other hand, the polyimide 12 may be exhausted prior to reaching the designated count value.
In the conventional polyimide coating device, when the predetermined frequency count is reached, the intermediate valve 18 automatically opens the discharge pipe 20. Since, the inside of the storage tank 10 cannot be seen, one cannot accurately determine the amount of polyimide 12 within the storage tank 10, and the discharge pipe 20 discharges all the remaining polyimide 12 in the storage tank 10.
On the other hand, when the polyimide 12 supply is exhausted before reaching the predetermined frequency count, a gas is introduced into the supply pipe 16. This gas remains within the supply pipe 16 even after the polyimide 12 is replaced. In such cases, when the polyimide 12 is supplied into the supply pipe 16, foams or small bubbles are generated in the polyimide 12 by the gas, thereby forming a polyimide film that is coated on a substrate, but which has the gas bubbles resident therein. When such a polyimide film is thereafter baked or thermal-treated, the bubbles burst, thus roughing the surface of the polyimide film and rendering the film useless.
In either of the above situations, manufacturing costs increase. In one case the remaining polyimide is wasted by discharging it prematurely, and in the other case, poor quality films are formed. In both cases, polyimide is unnecessarily wasted.